tracing: fix TracingController cleanup

This fixes an incorrect deletion of the TracingController instance, which in some environments could cause an error about an invalid pointer passed to free(). The TracingController instance is actually owned by a unique_ptr member of the platform, so calling platform::SetTracingController(nullptr) is the correct way to delete it. But before that, the TraceBuffer must be deleted in order for the tracing loop to exit; that is accomplished by calling TracingController::Initialize(nullptr).

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

tracing